Portable communication devices, such as cellular telephones, personal digital assistants (PDAs), electrical gaming devices, laptop computers, and the like, are configured to communicate over wireless networks. Accordingly, each such portable communication device relies on a receiver and a transmitter, typically connected to a single or common antenna, for sending and receiving data and control signals over the wireless network. Of course, the receiver and transmitter may be incorporated into a transceiver, having a receiver portion and a transmitter portion. In order to use the common antenna, often a duplexer is included to interface between the antenna and each of the receiver portion and the transmitter portion, so that the receiver portion is able to receive signals on a receive (downlink) frequency, and the transmitter portion is able to send transmit signals on a different transmit (uplink) frequency. The receive and transmit signals may be radio frequency (RF) signals, for example.
Various types of wireless network are implemented according to different communication standards, such as universal mobile telecommunications system (UMTS), global system for mobile communication (GSM), personal communications services (PCS), digital cellular system (DCS), international mobile telecommunication (IMT), and enhanced data rates for GSM evolution (EDGE). The communication standards identify separate bands for transmitting and receiving signals. For example, band 1 provides an uplink frequency band of 1920 MHz-1980 MHz and a downlink frequency band of 2110 MHz-2170 MHz; UMTS band 2 (PCS) provides an uplink frequency band of 1850 MHz-1910 MHz and a downlink frequency band of 1930 MHz-1990 MHz; UMTS band 3 (DCS) provides an uplink frequency band of 1710 MHz-1785 MHz and a downlink frequency band of 1805 MHz-1880 MHz; UMTS band 7 (IMT-E) provides an uplink frequency band of 2500 MHz-2570 MHz and a downlink frequency band of 2620 MHz-2690 MHz; and UMTS band 8 (GMS-900) provides an uplink frequency band of 880 MHz-915 MHz and a downlink frequency band of 925 MHz-960 MHz. Accordingly, a duplexer operating in compliance with a UMTS standard would include a transmit filter having a passband within the corresponding uplink frequency band, and a receive filter having a passband within the corresponding downlink frequency band.
The duplexer includes two band-pass filters having different passbands, thus preventing or reducing interference between the receive and transmit signals. That is, the duplexer includes a receive filter having a receive passband for filtering the receive signals, and a transmit filter having a transmit passband for filtering the transmit signals. The band-pass receive and transmit filters may include acoustic resonator filter elements, such as surface acoustic wave (SAW) or bulk acoustic wave (BAW) resonators, such as thin film bulk acoustic resonators (FBARs) or solidly mounted resonators (SMRs), for example, for filtering the receive and transmit signals. Generally, impedance matching circuits are needed to enable the duplexer to interface with the receiver and transmitter portions of a transceiver, respectively.
FBAR filters usually have single-ended input and output terminals for single-ended (unbalanced) signals, while the receiver or transmitter portions of the transceiver with which the FBAR filters interface typically have differential input and output terminals. An FBAR filter therefore relies on a balancing device, such as a balun (in addition to the impedance matching circuit), for example, to convert between single-ended and differential signals in order to interface with the transmitter and receiver portions of the transceiver. This adds to the size, cost and insertion loss of the component.
A balun suitable for such applications and frequencies typically include inductor-capacitor (LC) circuits, which require a number of inductors and capacitors. Such a balun may be referred to as a “lumped element balun.” The fewer inductors and capacitors included generally result in less space, and lower cost of the balun. Accordingly, design efforts have been made to achieve satisfactory balun functionality with the fewest possible LC elements, where LC element refers to a reactance (or susceptance) which is typically implemented as either an inductor or a capacitor. However, in general, conventional baluns require at least four LC elements, as found in a lattice-type balun, and typically more than four LC elements. Larger numbers of LC elements make implementation more difficult, typically lead to higher losses and require a larger area. Even in the lattice-type balun, the four LC elements must include two inductors and two capacitors. This may be problematic in technologies where one type of LC element cannot be implemented as easily or accurately as the other type of LC element, or is not conducive to space saving or high performance (e.g. high quality factor, less degradation by unwanted coupling effects, etc.) In addition, a lattice-type balun does not have a DC blocking capability between balanced and unbalanced ports. Accordingly, a lumped element balun is needed with no more than four LC elements, where at least three of the elements are either all capacitors or all inductors.